The present invention relates to a scanning probe microscope utilizing a quartz oscillator to control the position of a probe.
Known methods for position control in a scanning probe microscope include a method consisting of detecting a tunneling current, a method consisting of detecting evanescent light, and a method consisting of detecting an atomic force. One form of scanning probe microscope making use of a tunneling current for the control of a probe is a scanning tunneling microscope (STM). One form of scanning probe microscope in which evanescent light is employed for the control of a probe is a photon STM. However, limitations are imposed on samples capable of being measured. Therefore, principal applications lie in an atomic force microscope (AFM) where an atomic force is used to control the position of a probe and in a near-field scanning optical microscope (NSOM). One method of detecting an atomic force consists of detecting displacements of a probe by means of laser light. Another method makes use of variations in the current generated by a quartz oscillator.
A scanning probe microscope in which laser light is used to detect displacements of a probe is disclosed, for example, in Patent Unexamined Publication No. 50750/1994, entitled, "Scanning Microscope Including Force-Detecting Means", by Robert Erik Betzig. An example of a scanning probe microscope in which a quartz oscillator is used to detect displacements of a probe is disclosed in Appl. Phys. Lett. 66(14), 1995, pp. 1842-1844, by Kaled Karai et al. These instruments are outlined below.
FIG. 2 is a schematic view of the prior art "scanning probe microscope using laser light". The tip of an optical fiber 310 is machined into a tapering form 70. A sample stage 20 is placed on an XYZ stage 50. A sample 30 is set on the sample stage. Laser light from a lase light source 60 is introduced into the fiber 310 using a lens 150. The optical fiber probe 70 is vibrated parallel to the sample surface, using a fine displacement element 40. A horizontal force from the sample surface, or a shear force, acts on the tip of the probe. Thus, the state of the vibration of the probe varies. To measure the state of vibration of the probe 70, laser light (not shown) used for position control is directed at the tip, and the shadow of the probe 70 is directed through a pinhole 120 by a lens 90 and detected by a photomultiplier 110 of a position sensitive detector 80. The distance between the sample surface and the tip of the probe is controlled, using the fine displacement element 40, so that the shear force is kept constant, i.e., the rate at which the amplitude or phase varies is kept constant. The shear force drops rapidly with the distance from the sample. Utilizing this, the distance between the sample surface and the tip of the probe is kept constant on the order of nanometers. The sample surface is raster-scanned, using the XYZ fine displacement element 40. In this way, the topography of the sample surface can be measured on the order of nanometers.
FIG. 3 is a schematic view of main portions of the prior art "scanning probe microscope using a quartz oscillator". Indicated by 400 is an optical fiber probe, and indicated by 410 is a quartz oscillator. The optical fiber probe is bonded to the quartz oscillator with adhesive. The quartz oscillator is made to resonate, using a piezoelectric device (not shown) for vibrations. As the tip of the probe approaches the sample, a horizontal force from the sample surface, or a shear force, acts on the tip of the probe. Thus, the state of the vibration of the probe varies. The state of vibration of the quartz oscillator is measured by measuring electric charge generated by a piezoelectric effect of quartz. The distance between the sample surface and the tip of the probe is controlled, using a piezoelectric scanner (not shown) so that the shear force is kept constant, i.e., the rate at which the amplitude or phase varies is kept constant. The shear force drops rapidly with the distance from the sample. Utilizing this, the distance between the sample surface and the tip of the probe is kept constant on the order of nanometers. The sample surface is raster-scanned, using an XYZ fine displacement element (not shown). In this manner, the topography of the sample surface can be measured on the order of nanometers.
The prior art scanning probe microscope described above has the following disadvantages. In the scanning probe microscope using laser light, it is directed at the sample surface near the tip of the optical probe, and an image (shadow) of the tip of the probe is detected from the reflected light to detect the shear force. Therefore, the amount of reflected light is readily affected by the topography of the sample surface and by the reflectivity. Hence, it is difficult to measure the amplitude of vibration, and it is difficult to precisely measure the surface topography. Furthermore, it is not easy to align the laser light and so the data reproducibility has posed problems.
In the scanning probe microscope using a quartz oscillator, the portion where the quartz oscillator and the optical fiber are adhesively bonded together tends to be a microscopic region (e.g., a square region about 100 .mu.m square). It is difficult to perform the bonding operation. Furthermore, the characteristics of the quartz oscillator device are easily affected by the amount of adhesive, the hardness, the location at which they are bonded, and other factors. Thus, it is difficult to obtain an oscillator sensor with high reproducibility. For these reasons, it has been difficult to use the instrument in industrial applications. Where the probe is replaced, the quartz oscillator must also be replaced. This gives rise to an increase in the cost. In addition, reproducible surface topography measurement has been impossible to perform.